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The design the various parameters of the pond may be determined as indicated below:
(i) Surface Area:
The surface area of the pond may be determined by assuming a suitable value of organic or BOD loading for the pond expressed as kg BOD5/hectare/day. Since the amount of oxygen that can be produced by photosynthesis and the BOD that can be satisfied per unit area of the pond depends mainly on the quantum of sunlight falling on the pond surface, the permissible organic or BOD loading will depend on the minimum incidence of sunlight that can be expected at the pond location and also on the percentage of the influent BOD that would have to be satisfied aerobically.
The sunlight falling on the pond surface depends on the latitude of the pond site, its elevation above MSL, time of the year and sky clearance. Besides sunlight there are several other factors such as temperature, nature of sewage, type of algae and their radiation conversion efficiencies on which the organic or BOD loading of pond depends.
Many different methods have been developed for determining the permissible organic or BOD loading for ponds, out of which two methods are discussed here:
(a) In one of the methods the permissible organic or BOD loading for ponds have been related to the latitude of the pond location to aerobically stabilize the organic matter and keep the pond odour free. Based on the studies conducted, the National Environmental Engineering Research Institute (NEERI), Nagpur has given the values of the yield of photosynthetic oxygen in different latitude which are indicated in column 2 of Table 15.1. The Indian Standard IS: 5611-1987 recommends the organic or BOD loadings equal to the yield of photosynthetic oxygen as indicated in column 3 of Table 15.1.
The recommended organic or BOD loadings are for municipal sewage and are inclusive of the BOD of the settleable solids in the wastes. The values are applicable to towns at sea levels and where the sky is clear for nearly 75% of the days in a year.
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The values of organic or BOD loadings given in Table 15.1 may be modified for elevations above sea level by dividing by a factor (1 + 0.003 EL), where EL is the elevation of the pond site above MSL in hundred metres. An increase in the pond area has to be made when the sky is clear for less than 75% of the days. For every 10% decrease in the sky clearance factor below 75%, the pond area may be increased by 3%.
(b) In another method the permissible organic or BOD loading for ponds has been related to the ambient temperature on the assumption that temperature would depend on solar radiation. Thus based on field experience in warm climates the following expression has been given for the organic or BOD loading for ponds-
The designs based on the two methods given above, as well as other methods developed empirically, wherever possible should be checked against field experience in the region. When the ponds are intended to serve small communities such as institutions, or when they are located close to residences, it will be prudent to adopt lesser organic or BOD loadings than recommended so as to fully ensure absence of odours.
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The surface area of individual pond should not exceed 0.5 ha. Thus when the required surface area is less than 0.5 ha a single pond may be provided but when the required surface area exceeds 0.5 ha a number of small ponds working in parallel or in series may be provided.
The arrangement of multiple pond units is advantageous in case of repairs, maintenance and other unforeseen circumstances. Ponds in series have functioned more satisfactorily and are usually recommended for larger installations. To avoid anaerobic conditions in the primary pond, its area may be 65 to 70 percent of the total area.
(ii) Detention Period (or Detention Time):
The detention period of a pond should be adequate for the bacteria to stabilize the organic matter or achieve removal of BOD to the desired degree. The flow of sewage through a pond can approximate either plug flow or complete mixing, which are two extreme or ideal conditions.
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If BOD removal is described by a first order reaction, the pond efficiency is given by:
In the case of ponds located at very cold temperatures at high altitudes detention periods may have to be increased taking into account the decrease in the rate of biological activity at the lower temperatures. The detention period may be increased either by an increase in depth (up to 1.5 m) or by increasing the surface area of ponds.
(iii) Depth:
Having determined the surface area and detention capacity or volume of the pond, it becomes necessary to consider the depth of the pond only in regard to its limiting values. Shallow depths in facultative ponds will allow growth of aquatic weeds in the ponds while excessive depths will cause the entire contents in the ponds to turn anaerobic.
The optimum range of depth for facultative ponds is 1.0 to 1.5 m. When the depth determined from area and detention period works out lesser than 1.0 m, the depth should be increased to 1.0 m, keeping surface area unchanged. When the depth works out to be greater than 1.5 m, the depth should be restricted to 1.5 m and surface area increased to give the required detention period.
(iv) Sludge Accumulation:
The rate of sludge accumulation in facultative ponds depends primarily on the suspended solids (SS) concentration in the influent sewage. The reported rate of sludge accumulation in ponds treating municipal sewage ranges from 0.05 to 0.1 m3/capita/year. A value of 0.07 m3/capita/year forms a reasonable assumption in design. In multiple unit ponds operated in series, most of the sludge accumulation will be in the primary units.
Continued sludge accumulation in ponds over many years will cause:
(a) Sludge carry-over into the effluent,
(b) Development of aquatic weeds, and
(c) Reduction in pond efficiency due to reduction in the detention period. Hence facultative ponds require periodical dislodging at intervals ranging from 6 to 12 years.
(v) Bacterial Reduction:
Bacterial reduction in ponds is analogous to BOD reduction, except that the BOD removal rate constant is replaced by bacterial dieoff constant Kb and inputs and outputs are in terms of bacterial concentrations N1 and Ne, respectively. It is customary to use completely mixed conditions when calculating bacterial reduction. This gives a conservative design. Overall bacterial reduction in ‘n’ ponds of equal detention time ‘t’ in series is given by
A commonly used value of Kb for faecal bacteria at 20°C is 2.0 per day. The value of Kb at other temperatures may be calculated by-
Where Kb(T) and Kb(20) are values of the constant Kb at T and 20°C respectively.